Frank:  Since I'm a SF Bay sailor and it rarely gets below 35 in Alameda,ca - I thankfully don't have to worry about the raw water freezing (though manybe I should!). 

What I think about is a system to either dry out the raw water system (without injuring the impellor at startup) or filling the raw water system with fresh water or some other fluid. 

I don't like the thought of all the salt water just sitting in the heat exchanger eating away the zinc when the boat is not in use!!

But whatever system I think up, seems to be a lot of plumbing and have a high annoyance value. Is the RV fluid you speak of OK to exhaust into the water? or do you have to drain it out - save and reuse etc. 

The sweetest system would have a tank where the storage fluid would be pumped into the rawwater passages -- then pumped back for reuse when you want to use the boat.

Jay  

Bud,

I have a Universal 25XP with a heat exchanger. The RV stuff is supposed to be non-toxic and OK to dump in the river, but I was just trying to think of a way to avoid the annoyance of having to fill up the raw water system with antifreeze everytime I put the boat away. There is a company in Canada that makes a radiator heater with a fan that is not too expensive, and I was thinking of using something like that for a closed winter system (and getting a cabin heater in to the bargain.) I just wonder if it would dissipate enough heat.

Frank